Infrared Thermometer- Operation, Calibration, Pros, Cons, Application

An infrared thermometer is also sometimes known as the non-contact thermometer or the temperature gun or the thermal gun. So, as the name suggests, this thermometer does not make any physical contact while taking the temperature of any physical objects.

Design of Infrared Thermometer

1. Working Principle

Internal Structure of Thermal Gun
Internal Structure of Thermal Gun

Every object having a certain temperature emits infrared radiation. Similarly, the emission of infrared radiation from an object is directly proportional to the temperature of the object. Thus, by measuring the incoming radiation, the thermometer can measure the temperature of the object.

The incoming radiation goes to the lens of the machine. The lens converges the incoming radiation to the IR sensor. After that, the signal obtained from the IR sensor undergoes internal conversion. Finally, the converted value is displayed on the LCD display.

2. Distance to Spot Ratio (D:S Ratio)

Distance to Spot Ratio
Distance to Spot Ratio

To understand the basic concept of the D:S ratio, let us take an example of the building.

The building, having a certain temperature, will emit infrared radiation. Here ‘Distance (D)’ refers to the distance of the building from the thermal gun. Similarly, ‘Spot (S)’ refers to the diameter of the circular spot in the building where the thermal gun can measure. For example, the distance to spot ratio of 12 to 1, refers to the measurement of the 1-inch spot from the distance of 12 inches.

You can see in the figure that there is a direct relationship between distance and the spot. So, when the distance from the measuring object increases, the spot also increases and vice versa.  Thus, the ratio of distance to spot remains almost constant for the measuring instrument. You can see this value, in the specification of the infrared thermometer, that you buy from the market.

So, you need to be careful while taking the measurement of an object. If the distance increases, the spot also increases. But if the spot is greater than the object that you are trying to measure, then the thermometer can measure other irrelevant objects as well. Similarly, if the spot size is smaller than the size of the measuring object, you will not get the reading of the entire object. Thus, in both cases, there can be an error in the measurement.

To overcome the above problem, some thermal guns have been developed with an additional feature of the laser. The laser just shows the center of the spot.

How to Use Infrared Thermometer?

Press the switch of the thermometer. Then the display of the LCD will light up. Now point to the forehead of the person, whose temperature you want to take. Then press the switch again. Within a second you will hear an alarm which will indicate that the temperature measurement is completed.

Keep the following points in mind

  • Make sure to put a fresh battery within the thermometer.
  • You can change the setting of the alarm. So, put the setting to the alarm ON mode.
  • While taking the measurement, the distance between the forehead of the person and the tip of the thermometer should be from 5 to 15 cms. However, this is an approximate value and can differ from one model to another.
  • Set the machine to the body temperature mode for the measurement of body temperature. For the physical objects, set them to surface temperature mode.
  • In the case of forehead temperature measurement, the forehead should be free. There should not be a scarf around the forehead. Also, do not allow the hair from your head to cover the forehead.
  • If you are using the temperature gun for the first time, then make sure to check it. If it is faulty, then make sure to calibrate it.

Modes of Operation

You will find two types of modes for temperature measurement.

  • Body Temperature Mode:- This is the mode of operation for the measurement of the temperature of the human body. The temperature of the human body depends upon different factors such as age, sex, type, and the thickness of the skin.
Age (in years) Normal Temperature (0C) Normal Temperature (0F)
0 – 2 36.4 – 38 97.5 – 100.4
3 – 10 36.1 – 37.8 97.0 – 100.0
11 – 65 35.9 – 37.6 96.6 – 99.7
Above 65 35.8 – 37.5 96.4 – 99. 5

It also depends on the part of the body being measured.

Measurement Method Normal Temperature (0C) Normal Temperature (0F)
Rectal 36.6 – 38 97.8 – 100.4
Oral 36.5 – 37.5 95.9 – 99.5
Axillary 34.7 – 37.3 94.4 – 99.1
Ear 35.8 – 38 96.4 – 100.4

The temperature shown above is based on the measurement method.

  • Surface Temperature Mode:- This is the mode of operation for the measurement of the temperature of the physical objects.


When you are using a new temperature gun, you need to check it first. If the measurement of the temperature is faulty, then you need to do the calibration. For that, go through the video shown below.

Based on different models, there can be minor variations in the process of calibration. However, the basic concept remains the same.

Advantages of Infrared Thermometer

Some of the advantages of infrared thermometers are stated below.

  • The infrared thermometer gives an instantaneous reading (generally within a second). So, the measurement becomes fast. Similarly, the reading automatically gets reset. So, you can again take the measurement of another person (object).
  • They can store the data of the number of measurements that are taken. The storage capacity varies from the thermometer of one company to another.
  • The display shown on the LCD display of the temperature gun is numeric. So, in comparison to conventional thermometers, the reading process is much easier and quicker.  You can even take the measurement in a dark place. Similarly, the measurement can be taken in °C or °F.
  • There is even the provision of an alarm for abnormal temperature measurement.
  • If not used, the thermometers switch off after a certain period of time. Thus, it helps to save the life span of the batteries.
  • Since the method of measurement of the temperature of the patient does not include any physical contacts, the process is hygienic. There is no chance of cross-contamination.
  • Similarly, the non-contact thermometer can be useful in multiple applications. You can take the measurement of food, environment, solid surfaces, etc. You can even take the measurement of moving objects.

Disadvantages of Infrared Thermometer

Some of the disadvantages of infrared thermometers are stated below.

  • They are not suitable for the measurement of liquid and gases.
  • The accuracy of the infrared thermometer is less than the mercury thermometers. Similarly, if the measurement distance is not accurate, the chances of error increase.
  • You can only take the surface temperature of the object. The measurement of internal temperature is not possible.
  • In comparison to conventional thermometers, the price is relatively high.

Applications of Infrared Thermometer

Some of the applications of the thermometer are stated below.

  • For measuring the temperature of the solid objects.
  • For measuring the temperature of the moving objects.
  • For measuring the temperature of human beings, plants, animals, soil, etc.
  • For the detection of heat loss or gain, insulation breakdown, furnace, and duct leakage, etc.


You need to handle the temperature gun properly.

  • Do not expose the gun to heat, water, and direct sunlight.
  • Storage temperature should be within 10 to 40° C.
  • The relative humidity of the environment should be less than 85%.
  • Do not operate the infrared thermometer near the presence of strong electromagnetic fields such as cell phones.
  • Clean the lens of the thermometer using 70% ethanol. You can use Collins to clean the external surface.
  • If the display of LCD does not light up properly, there might be a problem with batteries. In that case, replace the battery with a new one, and try again.


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